Hematopoiesis is tightly regulated by a series of hormonal factors that stimulate the proliferation and differentiation of progenitor cells, as well as modulate the functional activity of mature effector cells. Human erythroid-potentiating activity (EPA) is a recently purified and molecularly cloned growth factor which stimulates the growth of erythroid precursors. The precise physiological role of EPA in the regulation of hematopoiesis is unknown. The proposed studies are designed to investigate the mechanisms of action of EPA on normal and neoplastic cells through characterization at the molecular level of the interaction of EPA with its receptor protein. Target cells bearing receptors for EPA as determined by binding of labeled 125I-EPA will be used as a source for purifying the EPA receptor and generating receptor antisera. Molecular clones encoding the EPA receptor will be obtained and used to determine the structure and sequence of the receptor protein and to examine its genomic organization in DNA. The cDNA clones will then be used to study the regulation and expression of the EPA receptor in normal and neoplastic cells. I propose to comprehensively analyze the EPA receptor on normal human peripheral blood and bone marrow cells, various human cell lines, and fresh neoplastic cells. Receptor numbers, binding affinity, and molecular properties of the receptor protein will be studied. Thus, significant differences may be seen in the quantitative or qualitative characteristics of the EPA receptor protein in normal and neoplastic cells which are important in the maintenance of the normal and transformed malignant state. Of particular interest is the possibility of tumor promoting activities or translocations or amplifications of the EPA receptor gene in specific neoplasias, in view of the association of some receptor proteins with oncogenes. Overall, these studies should yield new and important information on the in vivo regulation by EPA of hematopoiesis and its role in the pathophysiology of various disease states. Information gained from these studies should contribute to our basic understanding of the interactions of hematopoietic growth factors and their target cells. On a larger scale, this information should expand our current fundamental knowledge of hematopoietic cell growth and the interaction of growth factors and their receptors in normal and malignant states.